Why Doesn't MSVC Warn About Signed/Unsigned Integer Equality Comparisons?

MSVC Warning for Signed/Unsigned Integer Comparison

The following code raises the question of why MSVC doesn't issue a warning for equality comparisons between signed (int) and unsigned (unsigned int) integer values:

int a = INT_MAX;
unsigned int b = UINT_MAX;
bool c = false;

if(a == b) // no warning expected here
    c = true;

According to the C standard, when comparing signed and unsigned integers, the signed value is converted to unsigned. This conversion preserves the value for equality comparisons, as (-1 == -1) and ((unsigned)-1 == -1) are true. However, for other comparison operators like greater than (>) or less than (<), the conversion can lead to unexpected results. For example, (-1 > 2U) evaluates to true.

MSVC developers have made specific choices regarding the warning levels for these different operators:

• Equality comparisons (== and !=) do not trigger warnings because, for these operators, the conversion to unsigned preserves the result.
• Inequality comparisons (<, >, <=, and >=) do trigger warnings to alert users to the potential for unexpected results due to the conversion.

This approach ensures that warnings are raised for scenarios where the conversion could lead to surprising behavior, while avoiding unnecessary warnings for equality comparisons that maintain the expected result.

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